Apparatus for agglomerating powders

ABSTRACT

Apparatus for agglomerating powders and non-flowing fine particle minerals such as clay, silicas, and silicates involving means for establishing a minimum level of wetting agent content in the material to be agglomerated, a rotatably mounted motor driven drum for tumbling the material to be agglomerated over itself, spray nozzles located within the drum to add additional wetting liquid to the material within the drum, and conveying means for passing the material into the drum and discharge means for removing the agglomerated powder, which is now formed into pellets of an advantageous pellet size distribution, therefrom. The spray nozzles are situated to spray the wetting agent onto the material at the curl thereof. The establishing of a minimum level of wetting agent content in the powder, preferably in the range of 1-1/2% to 5%, is achieved either by incomplete drying of the material during wet processing or by the application of a wetting agent prior to introduction into the drum.

CROSS REFERENCE TO RELATED CASES

This is a continuation of application Ser. No. 450,037, filed Mar. 11,1974, now abandoned. And, which application was a divisional applicationof U.S. Ser. No. 186,113 entitled "Agglomerating Powders" filed Oct. 4,1971 by Robert B. Takewell, Paul W. Brandon, and Paul R. Odom now U.S.Pat. No. 3,803,283 granted Aug. 9, 1974.

BACKGROUND OF THE INVENTION

In general, the present invention involves agglomeration of finelydivided material and more specifically relates to an apparatus for wetpelletizing finely divided minerals and like substances into pellets ofan advantageous pellet size distribution which achieves optimum bulkdensity with free flowing characteristics for shipping, packaging, andstoring and reduces the amount of fines present while maintaining highdispersibility of the product.

Many powdery materials such as degritted and delaminated dry clay aredusty, non free-flowing and have a low bulk density of around 25 poundsper cubic foot. In order to induce such materials to flow from tanks,hoppers and chutes and other containers, vibrators, air pads, or hammershave to be used in order to prevent the fluffy material from caking orbridging. A finely divided material such as refined clay is very dustyin the dry state and, even with the best of air-tight containers andshipping vessels, much loss of the product and contamination of thesurrounding environment occurs.

Apparatus revealed by the prior art for agglomerating finely dividedpowders include pin mixer type pelletizers and rotating tilted panpelletizers. Materials pelletized in these devices achieve a fairly goodcompaction but produce pellets which are friable and substantiallyuniform in size. Also these devices produce fines and dust along withthe pellets so that the dust problem is only reduced and not eliminated.The pellets are denser than the powdery material but not of the optimumdensity for shipping and dispersion.

The prior art has suffered from the difficulty of obtaining a pelletsize distribution which allows the pellets to be placed in containersand storage vessels in the most compact manner. For example, pourdensities of kaolin clays pelletized by methods of the prior art such asthose disclosed by U.S. Pats. Nos. 3,446,218, 3,542,534, 3,460,195 and2,758,039, usually range from 50 to 55 pounds per cubic foot, whereasthe kaolin clays pelleted by the present invention exhibit pourdensities in the range of 60 to 70 pounds per cubic foot.

Other deficiencies of the prior art processes include the high rate ofattrition among the individual pellets in their containers which resultsin a large amount of undesirable fines. In order to reduce the attritionrate and prevent material loss and plant contamination due to excessivefines content, some of the prior art devices are designed to produce ahard pellet. While in some instances this serves to reduce the loss tofines, it always results in a pellet which is difficult to fracture anddisperse into the final product such as paint, paper, ink, rubber, etc.

Also, the prior art processes include methods which call for adding allof the moisture for pelletizing to the substance before it is introducedinto the pelletizer. This procedure even further reduces the flowabilityof the material and necessitates special conveying equipment such asscrew conveyors to move the material from the wetting chamber to thepelletizer. Since the wetted material, having from 5 to 20% or highermoisture level, is especially susceptible to caking in the moist state,it is difficult to convey smoothly.

Other existing devices add all of the water of pelletization to thematerial while it is in the pelletizer. This results in a dry powderyproduct being introduced into the pelletizer which causes a large amountof dust in and around the pelletizer arising from the powdery materialbecoming airborne. Also the completely dry material in the pelletizer ismore difficult to wet evenly than would be a slightly damp material justas a dry sponge is less absorbent than a slightly damp one.

SUMMARY OF THE INVENTION

Accordingly it is an object of this invention to provide new and highlyeffective apparatus for pelletizing powdered or finely divided materialand overcoming the deficiencies of the prior art noted above.

It is further an object of this invention to provide an agglomeratingdevice which produces an optimum pellet size distribution for maximumbulk density with free flowing characteristics of the agglomeratedmaterial.

It is also an object of this invention to reduce the amount of materialloss through attrition and fines in the pelletizer and in the shippingcontainer.

It is still a further object of this invention to provide a device foragglomerating loose material into pellets which are easily dispersed inthe final products in which they are used.

It is another object to densify powdered material into agglomerateswhich are free-flowing and non-dusting.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims taken in conjunctionwith the accompanying drawings.

The present invention overcomes the deficiencies of the prior art andachieves its objectives by providing novel apparatus for producing anagglomerated product which most efficiently utilizes shipping space byfilling up the voids which naturally occur between spherically shapedequal-sized particles packed in bulk containers such as bags, boxes,tanks, or railroad cars, and which exhibits free-flowing characteristicswith negligible dusting.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate the understanding of this invention, referencewill now be made to appended drawings of preferred embodiments of thepresent invention. The drawings should not be construed as limiting theinvention but are exemplary only.

In the drawings:

FIG. 1 is a diagram of the pelletizing process of the present invention.

FIG. 2 is a partially cutaway slide view of a rotating drum pelletizerof the present invention.

FIG. 3 is a cross-sectional view of the rotating drum pelletizer of thepresent invention taken at section line C-C of the drum, and in thedirection indicated.

FIG. 4 is an external view of the inlet end of the pelletizer drum ofthe present invention.

FIG. 5 is an external view of the outlet end of the pelletizer drum.

FIG. 6a is a front view of the spray nozzle used to inject the binderfluid into the pelletizer.

FIG. 6b is a side view of the spray nozzle.

FIG. 7 is a cross-sectional view of conventional pellets.

FIG. 8 is a cross-sectional view of pellets produced by the process ofthe present invention.

FIG. 9 is a top view of the rotating drum of this invention.

FIG. 10 is a schematic diagram of a further embodiment of the presentinvention.

FIG. 11 is a cross-sectional view of the drum taken at section line A--Ain FIG. 1 and in the direction indicated.

FIG. 12 is a cross-section of the drum taken at section line B--B inFIG. 1 and in the direction indicated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention is shown in FIG. 1 inwhich a storage tank 135 supplies a finely divided material such asclay, silica, silicates, etc., containing from 11/2 to 5 per centmoisture, to a conveyor system 2 through a supply line 4. The materialpasses through rotary feeder valve 46 and is then conveyed by conveyoror pin auger 2 to a right circular cylindrical rotating drum 1.

A moisture level of 11/2 to 5 per cent in the material to be pelletizedis obtained by incomplete drying of the material during wet processingor by adding moisture to the dry product prior to pelletizing. Forinstance in the clay industry, a raw kaolin clay, after being mined, issuspended in an aqueous slurry and degritted, classified and/ordelaminated and then dried. In applying the agglomerating process ofthis invention, the clay slurry is only partially dried to the extentthat it contains from 11/2 to about 5 per cent moisture, and then it isagglomerated according to the teachings of the present invention.

If the powdered material contains less than 11/2 per cent moisture thena sufficient amount of wetting liquid can be applied to it prior toentering the drum as for example, in pin auger 2, to bring its moisturelevel up to 11/2 to 5 percent. The finely divided material to beagglomerated, having the minimum amount of moisture in it, is added tothe pelletizing drum 1 and is further wetted by spray nozzle 8 as it istumbled. The pre-added moisture induces the finely divided material tomore readily accept the bulk of the wetting liquid added in the drum 1.The pre-wetting also results in a reduction of dust in the drum andconveying systems. It also provides an easily flowing blend which isless susceptible to caking.

The material in the drum receives the remainder of wetting liquidthrough pelletizer liquid supply line 56 which feeds the liquid to spraynozzle 8 located within the drum 1. The liquid is sprayed from spraynozzle 8 onto the material and the material is pelletized by therotating motion of the drum 1 and is passed through the drum outlet 14to the pellet conveyor 6. The drum 1 has removable manholes 66, shown inFIG. 2, opening into the drum to allow access to the inside of the drumfor maintenance. The manholes 66 can also be used to purge material fromthe drum when changing operation from one type of material to another orchanging grades of material.

Referring to FIGS. 2 and 4, the rotating drum 1 receives materialthrough drum inlet 15 which is a substantially cylindrical sectionpermanently attached to one end of the drum and is supported by floorsupports 16 and rotatably attached to the floor supports 16 by trunionbearings 13. A conveyor 2 such as a screw conveyor passes concentricallythrough the drum inlet 15 to bring material into the drum. A closure isobtained by lubricated bushings 17 between the conveyor 2 and the inlet15.

In the drum 1 perpendicular to the longitudinal axis are baffles 3 and7, FIG. 1, containing openings to allow the material to progress throughthe drum. The baffles divide the drum into three chambers to providethree different functions. Baffle 3 is located near the middle of thedrum length and forms the pelletizing chamber 39 at the inlet end of thedrum. Baffle 7 divides the remaining portion of the drum into twochambers, a polishing chamber 40, and a discharge chamber41 containingone or more lifter trays 42.

At the downstream end of the drum 1, the drum outlet 14 may contain atapered or right cylindrical discharge cone 43 concentrically locatedwithin outlet 14 to allow finished pellets to flow into discharge spout9. From the discharge spout 9, the pellets drop into pellet conveyor 6to be removed from the pelleting drum area.

A wetting liquid is supplied to spray nozzle 8 through line 56 and mayconsist of water and/or steam or air. Water is supplied from tank 36through line 5a and a rotameter 47. Steam or air enters throughsteam-air line 44 and steam-air valve 45.

In FIG. 3, the pelletizer drum 1 contains a bed of material 18 beingpelletized by continuously tumbling down the side of the shell. At thepoint 19 on the inner surface of the bed 18 when the material begins tobreak loose from the bed and tumble down towards the bottom of theshell, due to the increasing angle of repose, force of gravity, andforces arising from the drum rotation, the wetting liquid is mostadvantageously sprayed. This point 19 is referred to as the "curl" inthe tumbling bed of material 18.

The spray attitude angle 48 is the angle the spray makes with a verticalline through the axis of rotation of the drum 1. This angle ispreferably around 45°-50° but can range from 35° to 85° depending uponthe speed of rotation of the drum and the material being agglomerated.At a drum speed of 6 rpm, the preferable angle is around 45°.

The spray of liquid is directed at the material near the upstream end 49of the drum 1 but is not allowed to contact the end 49.

In FIGS. 2 and 4, the drum 1 has a concentrical inlet section 15 whichis permanently attached thereto and rotates with the drum. A nonrotatingconveyor system 2 passes concentrically through the inlet section 15 andis attached thereto by lubricated bushings 17. The conveyor system 1 issupported by support 20 passing beneath it.

FIGS. 2 and 5 disclose the pellet outlet 14 concentrically located inthe end 21 of the drum. Floor support 16 supports brackets 22 whichsupport the trunion bearing 13 mounted adjacent the drum outlet 14.

In FIGS. 6a and 6b, the spray nozzle 8 emits a flat, wide spray ofbinder fluid which forms a pattern with spray angle 23 and sprayinclination 24. Angle 23 is about 30° and spray inclination 24 is alsoaround 30°.

In FIGS. 7 and 8, the advantageous pellet size distribution isexhibited, with FIG. 7 showing the prior art uniform size pellets 26packed in bulk and FIG. 8 showing the pellets 27 of this invention alsopacked in bulk. The pellets 26 of FIG. 7 have many void spaces 25between them resulting in a waste of space and also allowing relativelylarge movements and attrition between the pellets during transportationand handling. FIG. 8 shows how the pellet size distribution of thepresent invention results in the voids 25 between large pellets 27 beingfilled with intermediate pellets 28, small pellets 29, and very smallpellets 30. Thus FIG. 8 shows how packaging and shipping space isutilized very efficiently and how the close packing of pellets reducesrelative movement between them and reduces attrition and fines.

In FIG. 9, the rotating drum 1 has permanently attached to the drum end21 a driven ring gear 10 which is constantly meshed with driving ringgear 11 on the shaft of driving motor 12. This arrangement providesrotation for the drum. Trunion bearings 13 rotatably support therotating drum. Pelleted material passes from the drum out outlet section14 and into pellet conveyor 6. Raw powdered material enters the conveyor2 at material inlet 34, is conveyed down the conveyor 2 which passesconcentrically through the inlet section 15 and into the drum 1. Theinlet section 15 is permanently attached to the drum and is alsorotatably supported by trunion bearing 13. The conveyor is supported byand attached to conveyor support 20. The shaft 35 of the conveyor isconnected to a pulley 33 which is driven by motor 31.

In typical operation, the material to be agglomerated is stored instorage tank 135 and is there allowed to deaerate by settling.

It has been found that deaerating the feed material prior toagglomerating it results in a more compact and desirable pellet and alsoreduces the amount of fines, since the pellets form faster in the drumwith the deaerated material than when it is not deaerated.

From the storage tank 135, the material is conveyed through line 4 toconveyor 2. It is metered by passing it through feeder valve 46. At theoutlet end 14 of drum 1, a supply line 56 receives wetting liquidthrough valve 47 from tank 36 and/on steam from line 44 and feeds thismixture to spray nozzle 8 in the rotating drum 1.

Since the material to be pelletized should contain from 11/2 to 5percent moisture for best results, a pre-wetting supply line 50 isattached to the feeder valve 46 in order to supply moisture to thematerial if it contains less than 11/2 percent moisture. If the materialis already sufficiently damp, valve 51 in line 50 can be closed toprevent excess pre-wetting.

The rotating drum is rotated at 6 rpm and material is fed throughconveyor 2 and into the drum at approximately 10 tons/hour. A wettingliquid, such as water or steam or both, is sprayed into the curl 19 ofthe bed of material 18 as it breaks loose from the drum wall and tumblesto the bottom of the drum.

When steam is used with water or by itself, it gives improved wettingability and results in better pellets which exit the pelletizer at ahigher temperature. The use of steam gives a greater spray pressure,finer droplets and mist, and less liquid surface tension. Air or othergases may also be used to aid in atomization. This results in a greaterpenetration into the bed of material and better dispersion of the liquidthroughout the material. Smaller droplets and better dispersion meansthat a given volume of liquid will go further and provide more evenwetting and a tighter control over the moisture content of the material.

The use of steam also provides a convenient method of adding heat to thepellets so that if they are to be dried later, less heat will berequired to dry them and they will dry more uniformly. Since heat andmoisture travel slowly through the dense agglomerated pellet, drying acool moist pellet results in drying the outer part of the pellet into ahard shell without removing all of the moisture near the center of thepellet. This process makes the pellet particularly susceptible tobreaking. When the pellet is formed at a temperature near thetemperature of vaporization of the wetting liquid, and is then dried,very little heat is required in the center of the pellet to vaporize thewetting liquid and remove it from the pellet. Thus the pellet driesuniformly from the inside out and is not case hardened as is thesituation when cool wet pellets are heated and dried. When steam is usedfor pelletizing, a desirable finished pellet temperature is from 65° to100° C.

Other suitable wetting liquids include one or more of the following:latex, oil, Petro A.G., silanes, stearates, starch, sizing agents,organic materials such as benzene, kerosene, alcohol, etc., organic orinorganic dispersing agents, dispersed and non-dispersed clay and thelike.

If a very hard pellet is desired, a binder additive such as molasses ororganic liquid can be added to the dry powdered material, or to thewetting liquid supply line. In the alternative, the binder may besprayed separately in the drum from the wetting liquid.

The drum can be of any dimension depending upon the volume of materialto be agglomerated. In one preferred embodiment of this invention thedrum has a length of 12 feet and an inner diameter of 10 feet. Athroughput of about 250 pounds of material per minute and 25 - 35 poundsof liquid per minute (approximately 3 gallons per minute) gives aneffective bed depth of about 8 to 18 inches.

A preferable bed depth when pelletizing clay using steam and water isaround 10 inches. At this depth, a particularly desirable pellet sizedistribution is obtained. A deeper bed depth results in more largepellets and less small pellets.

It is believed that the pellets form in layers or laminations, beginningwith a seed or nucleus and gaining a layer each time the pellet istumbled in the presence of loose wet material or passes through thespray. Thus a deeper bed results in each pellet remaining in contactlonger with available loose material. Also a deeper bed results ingreater pressure on the pellets as they pass near the bottom of the bedand the result is a harder more compact pellet which may be difficult todisperse.

The production rate of the agglomerating apparatus of the presentinvention depends upon the size of the drum, the critical speed ofrotation of the drum for the type of material used, and the moistureinput rate.

The critical speed of the drum appears to be the speed at which thecentrifugal force acting upon the material reaches such a magnitude thatit interferes with the normal cascading action of the material in thedrum. Instead of cascading down upon itself, the bed of material is heldagainst the drum wall longer until it breaks loose and arcs across tothe opposite side of the drum causing discontinuities in the normallycontinuous cascading mass of tumbling material. For the cylindrical drumof this embodiment, pelletizing clay with water and steam, the criticalspeed is determined by the formula: RS_(c) = √D/76.6, where RS_(c) isthe critical rotational speed and D is the diameter of the drum. With adrum diameter of 10 feet, the critical speed is determined to be 24.2rpm. In this embodiment of the present invention, a rotational speed of6 rpm was successfully utilized, which is well below the calculatedcritical speed. The production rate of the agglomerating apparatus couldthus be increased considerably by increasing the drum speed from 6 rpmto any speed below 24.2 rpm.

The drum can be heated near the outlet end in order to dry thecompletely formed pellets. The preferred maximum moisture level of thematerial in the drum is from 10 - 18 percent and this "moisture level"can be of any of the wetting liquids given above or any combination ofthem.

A coating such as epoxy resin or epoxy paint may be applied to the partsof the apparatus which come into contact with the material beingpelleted in order to eliminate the possibility of contamination of thematerial.

Pellets of "predispersed" clay may be produced in the present inventionby including a sufficient amount of a commercially known clay dispersantsuch as one of the polyphosphates or polysilicates, in the dry clay, ina prewetting tank, in the main spray 8, in the drum or in anycombination of these locations. A more uniform distribution of thedispersant is obtained by mixing it in the wetting liquid added throughsupply line 50 and feeder valve 46 and, subsequently, and in the sprayfrom nozzle 8.

The wetting liquid may be divided into two streams and sprayed from twodifferent spray nozzles at different locations in the drum. If thesprays are placed in tandem, an even greater reduction in dusting isachieved in the drum since the downstream spray wets down the dust notwetted down by the upstream spray.

A second spray may be used entirely for spraying a dispersant, binder,or any other additive onto the material with the first spray onlywetting the material. The dispersant or binder additive spray may beupstream or downstream from the wetting spray in the pelletizing drum.

In lieu of or addition to adding a wetting spray, a seeded bed may beutilized in pelletizing the material. Part of the pellets formed may bebroken down and recirculated to the upstream end of the drum to seed theincoming material.

In FIG. 10, an alternative drum configuration is shown which utilizes anintegral classification system built into the drum 66. In the dischargeend 41 of the drum 66, part of the drum shell has been replaced by bandsof screen material 56 and 57, passing entirely around the circumferenceof the drum. Screen section 56 contains a tight-mesh screen (such as,for example a 40 mesh screen) for allowing only fines or very smallpellets to drop out of the drum and be collected in funnel 58. Thescreen 57 is a large mesh screen (such as, for example, a 5 mesh screen)which allows the desirable pellets to drop out into funnel 59. Pelletsand lumps which will not pass through either screen are dischargedthrough outlet 60 into funnel spout 61. The large lumps collected infunnel 61 are passed to a crusher 62 which breaks the large pellets andlumps into small seeds and dumps them into conveyor means 63 whichcarries these seeds and the fines from funnel 58 into the drum and outdischarge spout 64 located near the incoming stream of finely dividedmaterial entering through conventional conveyor 65. The finely crushedlumps and small pellets serve as seeds or nuclei for the incomingmaterial and aid in the initiation of agglomeration of the material.

In using wet pelleting by adding a liquid, a typical moisture content ina material such as clay to obtain any pelletizing in the rotating drumappears to be on the order of 12 percent.

If heat is applied to the drum 1 in order to dry the pellets, theminimum amount of wetness in the final dried pellet should be above 1%with a preferable wetness level of 3 to 5 percent for maximumdispersibility of the material into other compounds such as paper orrubber. The temperature of the pellets leaving the heated drum should bearound 65° - 100° C. and the pellets should exhibit a spherically shapedform with a semi-polished surface.

A pellet formed in accord with the present invention when cut in halfreveals a small central nucleus and shells or laminations built uponthat nucleus. These laminations are believed to be due to the wettingspray contacting the loose material in the vicinity of the pellet. Eachtime the spray hits the area near the pellet, another lamination on thepellet results.

Heat can be applied to the drum by any conventional method such asexternal gas heaters or equivalents such as electrical resistanceheaters or jets or hot gases directed at the drum.

Finished and dried pellets exit the drum at outlet 14 and pass into apellet conveying system 6 whereby they are transferred to storagevessels.

While manufacturers usually require the pelleted material to besubstantially dry, some materials will flow freely when pelletized bythis process even though they may contain up to 18 percent moisture.

Since a clay pellet with less than 3 percent moisture is difficult todisperse for many applications such as paper, etc., the clay pelletsmust contain between 3 to 5 percent moisture to be acceptable to manypurchasers. Yet it was found that by pelletizing clay using the presentinvention, there is not any discernible reason for drying the pellets.In fact, it is advantageous not to do so. Pellets containing as high as18 percent moisture manufactured by the process of this invention wereextremely free flowing and do not adhere to one another when stored norbreak when handled normally. In addition, these free flowing moistpellets are easily dispersed in other products or into an aqueous slurrydue to their relatively high moisture content.

It has been found that the preferred pellet size distribution formaximum packing efficiency and minimum loss to fines through attritionis as follows:

    ______________________________________                                        Size Mesh Screen     % of Pellets                                             ______________________________________                                        + 5                  25 - 55%                                                 - 5 to + 10          30 - 55%                                                 - 10 to + 20         5.0 - 25%                                                - 20 to + 40         0.5 -5.0%                                                - 40                 less than 2%                                             ______________________________________                                    

The interpretation of this distribution is that from 25 to 55 percent ofthe pellets will not pass through a No. 5 mesh screen (largestopenings), 30 to 55 percent of the pellets will pass through a 5 meshscreen but not through a 10 mesh screen (slightly smaller openings), 5to 25 percent will pass through the 5 and 10 mesh screens but notthrough a 20 mesh screen, 0.5 to 5 percent will pass through the 5, 10,and 20 mesh screens but not through a 40 mesh screen, and less than 2percent will pass through all the screens including the 40 mesh. Thefraction passing through all the screens, considered as fines, may beconsidered undesirable because of the difficulty in handling theparticles, although the fines still consist primarily of small pellets,with very little dusty unpelleted material.

In pelleting powdered or finely divided materials such as clay orpigments, the above pellet size distribution range will provide a pelletdistribution that has high volumetric density with free flowingproperties.

This gradation of pellets exhibits a lack of any tendency to bridge andis extremely free flowing and free of dust. Although they are notunacceptable, pellets above 5 mesh in size and below 40 mesh in sizeshould be avoided in order to obtain the highest packing density andless attrition. The optimum range of pellet sizes is between 5 mesh and40 mesh and it is often desirable to have over 50 percent of the pelletsin this range.

While the prior art devices, such as pin mixers or rotating tilted pans,can pelletize within this range, the pellets produced are oftenundesirable because of their low density, low compaction and theiruniformity in size. These characteristics give the pellets a lower bulkor pour density than those of the present invention.

Another desirable feature of the present invention involves theball-mill type of action the larger pellets exert upon the small pelletswhen the pelleted material is being dispersed into slurry or anotherproduct. This ball-mill action aids in breaking down the small pelletsand therefore eases dispersion of the material into the final products.

In four different pelletizing runs, the above described methods andapparatus provided uniform semi-polished pellets of good dispersibilityand attrition resistance and well within the preferred range of pelletsize distribution. The table below gives the results of the pelletizingruns.

                                      TABLE I                                     __________________________________________________________________________    Mesh Size                                                                             Run 1 (%)                                                                             Run 2 (%)                                                                             Run 3 (%)                                                                             Run 4 (%)                                                                             Run 5 (%)                             __________________________________________________________________________     +5     34.8    30.6    42.1    47.0    39.1                                  -5 to +10                                                                             50.3    46.4    43.2    41.1    44.1                                  -10 to +20                                                                            13.2    19.9    12.8    9.4     15.0                                  -20 to +40                                                                            1.1     2.1     1.3     1.8     1.3                                   -40     0.6     1.0     0.6     0.7     0.5                                   Bulk Density                                                                          61.1lbs./ft.sup.3                                                                     65.4lbs./ft.sup.3                                                                     64.2lbs./ft.sup.3                                                                     64.3lbs./ft.sup.3                                                                     63.1lbs./ft.sup.3                     Material rate                                                                         250lbs./min.                                                                          250lbs./min.                                                                          250lbs./min.                                                                          250lbs./min.                                                                          250lbs./min.                          % Moisture                                                                            10.4    12.5    12.6    13.0    13.2                                  Drum rpm                                                                              6       6       6       5       6                                     Type Wetting                                                                          Steam   Steam   Steam   Steam   Steam                                 Liquid  and water                                                                             and water                                                                             and water                                                                             and water                                                                             and water                             Type Material                                                                         Clay    Clay    Clay    Clay    Clay                                  Drum Diameter                                                                         10 ft.  10 ft.  10 ft.  10 ft.  10 ft.                                Drum Length                                                                           12 ft.  12 ft.  12 ft.  12 ft.  12 ft.                                __________________________________________________________________________

The variables involved include the bulk density, the pellet sizedistribution, maximum and minimum size of the pellets, pellet hardness,and moisture level in the finished pellet. The bulk density and pelletsize distribution are directly related to the material through-put rate,drum size and speed of rotation, the type of material pelleted, theamount of wetting liquid added in the drum, the type of wetting liquidused, the tilt of the rotating drum, if any, the bed depth of materialin the drum, and the position of the spray in the drum. The maximum andminimum size of the pellets depend mainly on the type of material to bepelleted, the type of wetting liquid used, and the retention time in thedrum. Pellet hardness is dependent upon the type of material pelletedand the type of wetting liquid used. Pellet hardness is also dependentupon bed depth since the deeper the bed of material, the greater thepressure upon the pellets in the bottom of the bed and the greatercompaction attained. Pellet wetness depends upon the amount of wettingliquid added in the drum and the amount of heat applied to thedownstream end of the drum.

It has been found in general that, within limits, an increase in themoisture added has the effect of increasing the proportion of large sizepellets produced and thus increasing the bulk density up to the point ofdiminishing returns based on sizing and breakage factors.

An increase in the speed of revolution of the pelletizing drum has, ingeneral, the effect of increasing the number of small size pellets andtends somewhat to decrease the bulk density.

To control bed depth and retention time in the drum, the drum can betilted from inlet end to outlet end by raising one end or the other. Byraising the inlet end of the drum with respect to the outlet end,retention time will be shortened. Raising the outlet end higher than theinlet end lengthens the retention time.

Bed depth can be controlled closely by enlarging or closing off theopenings in the baffles within the drum. In FIGS. 11 and 12, the baffleswhich divide the drum into three compartments are visible. In FIG. 11,baffle 3 is shown having openings 53 placed equi-distant around thebaffle. These openings as well as the central opening 54 allow thematerial being pelletized to pass from the pelletizing compartment 39into the polishing compartment 40. Increasing the size or number of anyor all of these openings lowers the effective bed depth by allowing thematerial to pass down the length of the drum quicker. Bed depth is alsochanged by moving the openings radially inward or outward on the baffle,with bed depth decreasing as the openings are moved radially outward onthe baffle, towards the drum shell.

In FIG. 12, baffle 7 is visible, which is the baffle dividing thepolishing compartment from the discharge compartment. Bed depth in thepolishing compartment depends upon both the baffles, 3 and 7. If theopenings in baffle 3 are increased to lower bed depth in the pelletingcompartment, the opening 55 in baffle 7 must likewise be enlarged tomaintain a lower bed depth in the polishing compartment.

In the discharge compartment are lifters 42 which pick up the finishedpellets as the drum rotates and funnels them out through the outlet. Ifthe bed depth is to be lowered by enlarging the openings in baffles 3and 7 then the size or number of lifters 42 must also be increased.

The through-put rate in the drum can also be varied by tilting the drum,enlarging the baffle openings, and increasing the number of lifters. Aconstant bed depth can be maintained while the through-put rate isvaried just as in the above alteration, bed depth can be changed whilemaintaining a constant through-put rate.

Retention time is a function of drum length and diameter, tilt of thedrum, if any, and bed depth in the drum. Bed depth also depends upon therate of material input into the drum.

The advantages enjoyed by the present invention over the prior artdevices include the greater bulk densities attainable, a more freelyflowing pelleted product, an almost total absence of fines and dustresulting in less loss and less contamination, and a pelleted productwhich is easily dispersed in other compounds.

The material passing through the apparatus is completely pelletized andsubstantially all dust is removed from the material. While there arestill "fines" in the pelleted material, these are small pellets ratherthan dust and are not objectionable. No sizing of the pellets isrequired since the finished pellets of this invention fall within thedesired range for optimum packing density and ease of dispersion. Thepresent invention eliminates the need for classifying the pellets andeliminates the need for dust removal apparatus.

When loaded in railroad hopper cars, a clay pelleted by the process ofthis invention flowed out through the discharge spouts at the bottom ofthe hopper cars at the rate of 1 ton per minute, without the aid ofvibrators, air pads, or hammers. There was a total absence of dustduring the unloading. A non-pelletized clay was unloaded at a rate ofone-sixteenth of the above using vibrators and air pads.

When loaded in boxcars, material such as clay must be unloaded by frontend loaders which are basically small tractors with raisable scoops onthe front. Unloading a clay pelleted by this invention was quick andconvenient while unloading a non-pelleted clay was slow and dirty due tothe heavy dusting occurring.

In addition, the boxcar containing the pelleted clay contained more thantwice as much clay per unit volume than did the boxcar of unpelletedclay, meaning lower shipping costs for the pelleted clay.

Although a specific preferred embodiment of the present invention hasbeen described in the detailed description above, the description is notintended to limit the invention to the particular forms or embodimentsdisclosed herein, since they are to be recognized as illustrative ratherthan restrictive and it will be obvious to those skilled in the art thatthe invention is not so limited. For example, the drum of the presentinvention can be level or tilted. The drum may be of any convenientlength and diameter. Water can be used as a wetting liquid as can steamor both; various binder additives such as molasses can be added to thewater, and other wetting liquids such as oils or organic liquids can besubstituted for water. The drum can also be used for dry pelletizationusing the seeded bed principle whereby a portion of the finished pelletsare crumbled and refed back into the unpelletized material to provideseeds or nuclei for newly forming pellets. The inner surface of the drummay be smooth, rough, or may have fins or lifters attached to it toagitate the bed of material. Also, more than one spray can be utilizedin the drum and a commercially available dispersant can be added to thematerial or to the wetting liquid or to a separate spray to provide a"predispersed" product, such as predispersed kaolin clay. Additionalclay of the same or other types may be added in slurry form to thatbeing pelletized by the spray. The drum can be heated near its outletend by gas or electric heaters or can be used without heating. As willbe noted the present invention substantially reduces the "dustingproblem". In the absence of the present invention it is not uncommon tolose 3 to 6 percent of the material shipped to the atmosphere. Thisinvention has particular advantages for export shipments where densityis of the greatest importance and multiple handling is required. Theinvention is declared to cover all changes and modifications of thespecific example of the invention herein disclosed for purposes ofillustration, which do not constitute departures from the spirit andscope of the invention.

We claim:
 1. Apparatus for agglomerating finely divided materials suchas clay, silica, or silicate pigments into a range of pellet sizesproviding optimum density, packing efficiency, and attrition resistance,comprising:a. a rotatable drum for receiving finely divided material andtumbling it until it is agglomerated; b. pellet initiation means locatedwithin said rotatable drum for spraying liquid wetting agent, in a flatwide spray, on the finely divided material for encouraging it toagglomerate; c. input conveying means passing into said rotatable drumfor introducing finely divided material into said drum, said inputconveying means including a screw conveyor comprising a non-rotatableshell and a rotatable shaft, and being connected to said rotatable drumby lubricated bushings; d. inlet means opening into one end of said drumand attached to said conveying means for receiving said finely dividedmaterial to be agglomerated; e. outlet means opening into the oppositeend of said rotatable drum for receiving the agglomerated material fromsaid drum; f. pellet conveying means located at said outlet means forconveying the discharged agglomerated material away from said drum; g.pre-wetting means attached to said input conveying means for adding from0 to 5 percent wetting liquid to said finely divided material; h. firstmotor means attached to said rotatable drum for providing rotary motionto said drum; i. second motor means attached to said input conveyormeans for providing conveyor drive power to said input conveyor means.j. a first baffle within said drum for dividing said drum intocompartments; k. a second baffle within said drum between said firstbaffle and said outlet means for further dividing said drum intocompartments; and, l. a plurality of lifter trays located radially insaid drum between said second baffle and said outlet means for liftingthe pelleted material and funneling it out of said drum.